Expansion bolt for excavation roof



July 21, 1970 S. W. ZOLDOK EXPANSION BOLT FOR EXCAVATION ROOF Filed May 14, 1968 Stephen w. Zoldok INVENTOR. BY W United States Patent r 3,521,522 EXPANSION BOLT FOR EXCAVATION ROOF Stephen w. Zoldok, E. 1425 40th St., Spokane, Wash. 99203 Filed May 14, 1968, Ser. No. 729,004 Int. Cl. F16b 13/10 us. Cl. ss 79 3 Claims ABSTRACT OF THE DISCLOSURE A bolt for securing the roof of an underground excavation comprising companion segments of a wooden cylinder having means for expanding the peripheral size radially during relative axial movement of the segments.

My present invention relates to mine bolts and more particularly to wooden expansion bolts for securing the roof of an underground mining excavation or the like.

In any underground excavation, some of the downward acting forces are transmitted to the pillars or walls of the excavation. The remainder are absorbed by the rock immediately overlying the excavation. Where the overlying rock lacks sufficient strength to support itself over the, span between the pillars or walls, the roof begins to sag and tension cracks appear. Continuous sagging causes the cracks to work their way upwardly and thereby the roof structure progressively loses strength. The lower strata being cracked and weakened, if unsupported, starts to fall. Depending on its structure, the roof may fall all at once, or in stages, covering a period of days or weeks. As the roof falls, more downward forces are transferred to the pillars or walls and less to the rock overlying the excavation.

In time, sufficient rock has fallen so that all of the remaining downward forces are transferred to the pillars or walls and none to the roof structure. At this point, if there are no large faults or slips in roof structure, no more roof will fall. This is the principle of the arch, well known to mining and construction men.

How much material will fall from the roof is a function of the strength of the rock or material above the excavation and the width of the opening. If the material above the excavation is sufficiently strong to be consolidated or stratified and is free of large slips or cracks, the vertical extent of rock that will fall should not exceed more than one-third of the width of the excavation at its center.

First experiments with mine bolts were conducted during 1949 at the Rio Verde Mine, Nolton Cole Corporation. Nortonville, Ky., reported by Sterling S. Lanyear, Jr., in an article entitled Wooden Pins for Mine Roof Control, in the American Mining Congress, Coal-Mine Modernization Year Book, 1950, and it was found that the utilization of wooden pins reduced the cost of anchoring the roof of a mine by approximately 80% of the cost of using conventional steel bolts. In the intervening time, however, the use of wooden pins or bolts has been ignored and steel torque bolts almost exclusively have been employed.

Anchoring strength of mine bolts in diiferent kinds or rock vary widely. Strength may vary considerably even in the same kind of rock. Some hard, igneous rocks will provide an anchor of 30 tons and possibly more. It has been determined, over the years, that roof bolts should be capable of providing a minimum anchor of from 3 to 5 tons and this has become a standard in the field.

The preferable way to utilizing mine bolts is to cause all mine bolt anchors to extend at least 6 or 12 inches above the arch line. In this manner an estimatable weight of rock would be suspended from solid ground "ice which would not fall, by reason of its being supported by the walls or pillars. The spacing of the mine bolts would thus be calculated in accordance with the following example:

An excavation which is 15 feet wide, would likely have an arch line at the center, about five feet vertically from the lower surface of the roof. The anchor bolts would be placed on four-foot centers in rows spaced four feet apart. The volume of rock below the arch line is approximately or cubic feet. Where the rock weighs approximately 150 pounds per cubic foot, we calculate a total of 22,500 pounds. This weighth, distributed over three bolts spaced four feet apart, requires each bolt to support approximately 3.75 tons of material.

Where the excavation is larger, and it is not practical to use bolts of a length sufficient to extend above the arch line, the bolts are still practical by utilizing them to pin together the cracked or stratified material beneath the arch line, utilizing the beam principle in which the material pinned together may be increased as much as three or four times in its strength for forming a supporting beam.

The ability of a rock bolt to support the roof of an excavation is directly related to its resistance to displacement by the weight it is supporting. The conventional rock bolt is secured in much the same manner as a blind rivet and includes an expansion shell which is inserted into a drill hole and then by use of a wrench, is expanded into frictional engagement with the side walls thereof.

In the Engineering and Mining Journal of May 1962, an adhesive bolt, developed in Germany, was disclosed and defined as costing 15% to 20% more than the conventional steel bolt. There have been other attempts at improving and simplifying the devices and means for bolting mine roofs, such as that disclosed in Letters Patent 2,748,594, but these have not been commercially accepted for one reason or another.

My present invention lies in the provision of a wooden roof bolt which will effectively support three tons or more, is inexpensive to manufacture and very simple to apply, thus permitting the function of bolting a mining roof to be accomplished at much less expense than is the case with conventional steel rock bolts.

The foregoing and other objects and advantages of the present invention will become more apparent and meaningful during the course of the following specification when considered in association with the accompanying drawings wherein a preferred form of the invention is graphically illustrated. It is to be understood, however, that the drawings are illustrative only and are not intended to limit the scope of the invention. It should also be understood that various changes in construction may be resorted to in the course of manufacture without in any way departing from the spirit of the invention which is to be understood only in accordance with the appended claims. Furthermore, it is to be understood that while the invention is described in one particular association, it is not my intention to unnecessarily limit the applicability of the invention, but I desire to reserve to myself the claimed invention for every use of which it is now known or subsequently discovered to be susceptible.

Other advantages and features of this invention will become apparent from the more detailed description following in which like reference numerals are employed to designate similar parts in the accompanying drawings,

wherein:

FIG. 1 is a lateral sectional view of an underground 3 excavation, showing an application of the wooden roof bolts of this application;

FIG. 2 is an enlarged longitudinal view, partially in section, showing a wooden roof bolt inserted in a drilled hole;

FIG. 3 is a view similar to FIG. 2 and showing the roof bolt expanded into bolting position;

FIG. 4 is a lateral cross section, upon an enlarged scale, of one roof bolt; and

FIG. 5 is a view similar to FIG. 4 of a modified structure.

Referring now more particularly to the drawing, FIG. 1 discloses an excavation E having a floor F bounded by side walls or pillars WW supporting the roof R. The unsupported downward forces on the roof R, which are not transferred to the pillars or walls W, are absorbed by the rock immediately overlying the excavation and as this roof begins to sag, tension cracks C begin to form. As the sagging process increases, the tension cracks migrate upwardly to approximately the arch line A. As soon as practicable after the excavation E is formed, the approximate weight of material below the arch line A requiring support is calculated, and then the number and spacing of roof bolts, to prevent the sagging and formation of tension cracks C, is determined thereby. A rock drill is used to form openings D, which in this disclosure will be described as substantially 2 inches in diameter, by using a two-inch drill bit, but which, obviously, may be of other sizes larger or smaller as desired. Preferably, the openings D are formed to extend from 6 to 12 inches or more above the arch line A as indicated by D and D respectively. But, where this is impractical, the openings D may be provided which just reach or terminate short of the arch line A.

Having formed the openings D, D and/or D", I insert into each of the openings an expansion bolt, indicated in its entirety by the numeral 10, which bolt comprises an elongated wooden dowl rod of circular section, having two segments 12 and 14. The segments are disjointed longitudinally of the bolt 10 and have a plurality of contiguous allochiral axially angular flat faces 16 and 18. The angle of the faces 16 and 18 is the same on both segments 12 and 14, but may vary in accordance with the inherent resistance to compressability of the particular type wood employed. My empirical observations have indicated that the trees of the genus Larix and Abies, which have a tough, durable wood, are very effective in the practice of this invention. With the aforementioned woods, I prefer to provide a series of contiguous faces which are disposed at an angle of four to thirty or diverging approximately l3.3 from the axis of the dowel.

The expansion bolt 10 is inserted into the o ening I) with the allochiral faces 16 and 18 in full face to face relationship as seen in FIG. 2 of the drawing and thus the segment 14 becomes the static segment and the segment 12 becomes the driven segment. It will be seen that the driven segment 12 extends a predetermined distance below the lower end of the segment 14, which distance is slightly less than the axial dimension of each flat face 16 or 18. In practice, each flat face is 3 inches long and terminates with a substantially right angle wall 19 which is substantially 0.4 inch. The extension in this relationship is 2.5 inches. It will also be seen that the segment 14 extends upwardly above the upper end of the segment 12, the same 2.5 inches, thus providing for 2.5 inches of relative longitudinal movement.

Since the plural allochiral flat faces are companion, the force P, which may be supplied by a hydraulic cylinder or other force producing device, to the segment 12, causes it to move axially of the segment 14 and the angular flat faces 16 and 18 coact to separate the segments 12 and 14 converting the axial force F to an expansion force increased proportionately by the ratio of angularity of the faces 16 and 18 to the axial movement.

The coeflicient of riction f was calculated to be .12

4 during my tests. An axial force of 10.3 tons was applied axially of the driven segment 12 at which time both segments began to move even though the force F was applied only to the driven segment 12. By trigonometric calculations it was determined that a force, perpendicular to the direction of movement, of 86 tons was created.

A cylinder having a 2-inch diameter and 36 inches long has 226 square inches of surface a, but since the pressure, perpendicular to the movement is applied on a plane perpendicular to the flat faces 18 and 16, not all of the surface area of the cylinder applies pressure to the surrounding cylinder of the bore hole B. To determine the area which frictionally engages the rock, the equation Fa/af was employed. This indicated that an average of 86 square inches of the expansion bolt were actually in frictional engagement with the cylindrical walls of the opening D; that is, 38% of the circumference, since the circumference of a 2-inch diameter cylinder is 6.28 inches, a total of 2.39 circumferential inches is in contact with the opening D which divided in half shows that 1.195 circumferential inches of the opposed segments is in contact or approximately 66 of arc. It therefore becomes obvious that a wooden expansion bolt having a larger circumference would support a greater weight as would also an expansion bolt of the same 2-inch diameter, but having greater length.

In the tests where the expansion bolt was 36 inches in length and 2 inches in diameter, it required 10.3 tons to move both segments 12 and 14 in the opening D. It can be calculated therefore, that for each inch of length, the 2 inch wooden expansion bolt is able to support .286 tons.

Since in many cases the rock immediately overlying the excavation is in layers or strata, there is the possibility that relative movement between the strata could shear the wooden expansion bolt 10. I, therefore, provide a reinforcing bar 20, which may extend for the full length of the driven segment 12 or a portion thereof as desired, and this bar 20 may be admitted by forming an axially parallel bore 22 in the segment 12 and then press the bar 20 into the bore 22. Alternatively, it may be applied by forming an axially parallel mortise 24 and admitting the bar 29 by moving it perpendicularly to the axis of the bar into the mortise 24.

It has been found by experimentation that the reinforcing bar 20 is of great value, even where there is no danger of shifting of the strata because it reinforces the driven segment 12 near the lower end where the force F is applied. To illustrate; when the force F is 10.3 tons at the end of a 36 inch expansion bolt 10, 6 inches from the end or 30 inches from the inner end, the force is 8.57 and it has been found that within this first few inches the driven segment 12 is most likely to fracture. This failure is probably due to the angular force generated by the lateral movement of the segment 12 during the longitudinal application of force. The reinforcing bar precludes this fracturing.

Having thus described my invention, I desire to secure by Letters Patent of the United States the following:

1. An expansion bolt for securing the roof strata of a subterranean excavation, comprising:

an elongated wooden dowel rod of circular section having two segments disjointed longitudinally thereof and constituting a driven and a static member with the driven member projecting at one end beyond the static member; each segment having means at the said disjointure adapted to cooperably expand the said rod equally along its effective length when the driven member is moved longitudinally with respect to the static member; said driven member having embedded therein a longitudinally extending reinforcing bar of material stronger than the wood, said driven member being of sufficiently greater section to accommodate said bar.

5 6 2. The invention according to claim 1 and further 2,014,892 9/ 1935 Graham et a1. characterized by: said reinforcing bar extending at least 3,301,123 1/ 1967 Worley 85-79 through the projecting portion of said driven member. 2,651,962 9/ 195 3 Hammond.

3. The invention according to claim 1 and further FOREIGN PATENTS characterized by: said members having companion allochiral axially angular fiat faces at the disjointure con- 5 127,529 5/1948 Australia; structed and arranged to constitute the said means. 24,383 9/ 1900 Great Brltaln- 711,324 6/1954 Great Britain. References Cited UNITED STATES PATENTS 1O EDWARD C. ALLEN, Primary Examiner 1,438,855 12/1922 Rawlings 85--82 

